1. Field of the Invention
The present invention is related to phase locked loops for providing an output signal depending on a periodic input signal. The present invention is further related to phase locked loops having analog or digital phase control loops.
2. Description of the Related Art
An analog phase locked loop circuit usually includes a frequency/phase detector, a charge pump, a loop filter and a voltage controlled oscillator as well as a feedback unit for feeding back the output signal to the frequency/phase detector. The frequency/phase detector is usually provided to compare the input signal with a feedback signal supplied by the feedback unit such that a leading/lagging information is obtained. If the feedback signal is lagging with respect to the input signal, the leading/lagging information results in an increased voltage supplied to the voltage controlled oscillator in such a way that the output signal is increased in its frequency. If the feedback signal is leading with respect to the input signal, the frequency of the output signal is reduced. Thereby, the frequency of the output signal is controlled to be constant at the frequency of the input signal or, if the feedback unit comprises a feedback divider, determined by a multiplication of the division value given by the feedback divider and the input frequency.
As the discrimination of the frequency/phase detector is poor due to the mere detection of a leading and lagging of the feedback signal, a phase locked loop according to the prior art produces an inherent noise as a result of a frequency toggling around the nominal output frequency.
The same is true for a digital phase locked loop circuit as the frequency/phase detector is designed similarly and therefore also has a discrimination which merely provides a leading/lagging information which substantially results in an increase or decrease of the output frequency. This also results in an inherent noise of the output frequency depending on the poor discrimination of the frequency/phase detection.